An apparatus as described in the opening paragraph is generally known, and can be applied in the preparation of coffee, tea, soup, etc., from particulate material located in the brewing chamber, such as ground coffee beans, tea leaves or soup powder, respectively. Since the taste and color of the beverage are generally determined by the employed quantity of particulate material per unit volume of water, it is desirable to provide the apparatus with some means of indicating the volume of water placed in the reservoir. On the other hand, it is also useful to know how many cups of beverage are contained in the jug at any particular time.
To meet these requirements, a wall of the jug is usually provided with a graduated volume scale. If the jug is used to transfer water from a faucet to the reservoir, then this scale serves to measure the volume of water drawn from the faucet, in accordance with the volume of beverage which the user wants to brew on any particular occasion. The same scale is then additionally intended to serve the purpose of indicating the volume of hot beverage contained in the jug after the brewing process. It is particularly useful if such a scale is directly graduated in terms of "cups" of liquid, rather than in regular fractions of a liter or pint, for example.
However, the provision of such a simple scale can lead to inherent inaccuracy in the preparation of the beverage. This is caused by the fact that the volume V.sub.f of hot beverage derived from the brewing process is generally significantly lower than the volume V.sub.i of faucet water administered to the reservoir to begin with. The ratio V.sub.f /V.sub.i will hereinafter be referred to as the liquid yield ratio R.sub.L of the apparatus, and has a typical value of about 0.8 in the case of a coffee maker. This relatively low value is caused by significant water losses resulting inter alia from steam production, condensation in the brewing chamber, retention of water in the particulate material from which the beverage is brewed, etc. The volume of faucet water initially indicated by the jug scale therefore does not correspond to the volume of derived beverage subsequently indicated by the jug scale.
In this scenario, the user must himself make allowances for water losses in the brewing process. In order to accurately brew a quantity Q of beverage, he must place a quantity Q/R.sub.L of water in the reservoir, and must therefore not only know the value of R.sub.L for the particular apparatus concerned, but must also perform a calculation each time he uses the apparatus. In an attempt to circumvent this problem, one might attempt to provide the wall of the jug with two separate graduated scales, which differ in pitch by a factor R.sub.L. However, this can be confusing, particularly to elderly users, and leads to compounded complication if each scale is required to be graduated both in cups and in mugs, as is commonly desired.